1. Field of the Invention
The present invention relates to an arc tube of a discharge bulb which is to be used as a light source of a headlamp for a vehicle, and a method of manufacturing the arc tube.
2. Description of the Related Art
In recent years, an arc tube has often been used as a light source of a headlamp for a vehicle because it can carry out irradiation with a high luminance. As shown in FIG. 12, an arc tube to be used in a headlamp for a vehicle generally comprises an arc tube body 104 formed of quartz glass in which a pinch seal portion 104b is provided on both sides of a light emitting tube portion 104a forming a discharge space 102. The arc tube includes a pair of electrode assemblies 106 having a tungsten electrode 108 and a lead wire 110 coupled and fixed to each other through a molybdenum foil 112. Each electrode assembly 106 is pinch sealed with the arc tube body 104 in each pinch seal portion 104b such that the tip portion of the tungsten electrode 108 is protruded toward the discharge space 102. A metal halide is enclosed in addition to an inactive gas and mercury in the discharge space 102 of the arc tube in order to enhance a color rendering property during lighting.
Since the arc tube body 104 is formed by performing a thermal process on a quartz glass tube, an almost wedge-shaped slit 102a is inevitably formed around each tungsten electrode 108 on both ends in the axial direction of the discharge space 102. In each slit 102a, a temperature during the lighting of the arc tube is lower than that in the other portions of the discharge space 102. Therefore, a metal halide is easily deposited on the slit 102a. As shown in FIG. 12, the metal halide 114 deposited on each slit 102a does not contribute to light emission during the lighting of the arc tube. Therefore, there is a problem in that the light emitting color of the arc tube is changed to be a different color from a predetermined color. Moreover, when the amount of the metal halide 114 deposited on each slit 102a is increased to some degree, the metal halide which can be effectively used for the lighting of the arc tube becomes insufficient so that there is also a problem in that lighting failures are caused.
The invention has been made in consideration of such circumstances and has an object to provide an arc tube capable of effectively suppressing the change of a light emitting color and the generation of lighting failures due to the deposition of the metal halide on the slit. In order to achieve the object, the present invention includes a structure of an arc tube body that reduces the volume of a slit, thereby decreasing the amount of deposition of a metal halide.
More specifically, in a first aspect of the invention, an arc tube may comprise an arc tube body including a light emitting tube portion forming a discharge space, a pinch seal portion disposed on sides of the light emitting tube portion, and a neck portion disposed between the light emitting tube portion and the pinch seal portion. Further, a pair of electrodes are pinch sealed with the arc tube body at the pinch seal portion such that a tip portion of each of the pair of electrodes protrudes towards the discharge space,
wherein each of a pair of opposing pinch seal surfaces of the pinch seal portion include a general portion and a step-down plane portion formed to have a substantially planar shape in step-down with respect to the general portion, and
wherein an axial distance from the neck portion to the step-down plane portion is 1 mm or less.
Moreover, in a second aspect of the invention the arc tube may comprise an arc tube body including a light emitting tube portion forming a discharge space, a pinch seal portion diposed on sides of the light emitting tube portion, and a neck portion disposed between the light emitting tube portion and the pinch seal portion. Further, a pair of electrodes is pinch sealed with the arc tube body at the pinch seal portion such that a tip portion of each of the pair of electrodes protrudes towards the discharge space,
wherein an axial distance measured in a direction going away from the light emitting tube portion from the neck portion to a tip of a substantially wedge-shaped slit formed between the arc tube body and at least one of the pair of electrodes is 0.5 mm or less.
Further, a method of manufacturing an arc tube is disclosed in which the method comprises forming an arc tube body of quartz glass which is provided with a pinch seal portion on both sides of a light emitting tube portion forming a discharge space and a neck portion between the light emitting tube portion and the pinch seal portion, and a pair of tungsten electrodes which are pinch sealed with the arc tube body in the pinch seal portion such that a tip portion is protruded toward the discharge space,
wherein the pinch seal is carried out by using a pair of pinchers having a step-up plane portion for forming a step-down plane portion in the pinch seal portion and causing an edge on the light emitting tube portion side in the step-up plane portion of each of the pinchers to abut on the arc tube body in a position from a position where the neck portion is to be formed by an axial distance of 1 mm or less.
In the discussion of the invention, a xe2x80x9ctungsten electrodexe2x80x9d is disclosed which includes an electrode that contains tungsten as a principal component.
Further, the xe2x80x9caxial distancexe2x80x9d discussed herein indicates a distance in the axial direction of the arc tube.
The xe2x80x9cneck portionxe2x80x9d discussed indicates a narrowed portion between the light emitting tube portion and the pinch seal portion and the position in the axial direction of the arc tube is specified as the most narrowed position.
In the first aspect of the invention, if the xe2x80x9cstep-down plane portionxe2x80x9d is formed to have an almost planar shape in step-down with respect to the general portion, a specific structure such as a contour shape thereof or the amount of step-down with respect to the general portion is not particularly restricted.
The setting of range of the xe2x80x9caxial distancexe2x80x9d in each invention may be applied to both of the pinch seal portions on both sides of the light emitting tube portion or may be applied to only one of them.
In the structure described above, the arc tube according to the first aspect of the invention has such a structure that each of a pair of pinch seal surfaces of the pinch seal portion formed in the arc tube body which are opposed to each other includes a general portion and a step-down plane portion formed to have an almost planar shape in step-down with respect to the general portion, and an axial distance from the neck portion to the step-down plane portion of the pinch seal surface in the pinch seal portion is set to have a value of 1 mm or less.
More specifically, since the axial distance from the neck portion to the step-down plane portion of each pinch seal surface in the pinch seal portion is very short, a sufficient pinching pressure can be applied to the tungsten electrode up to a portion close to the tip portion thereof during the pinch seal. As a result, it is possible to reduce the volume of the almost wedge-shaped slit formed on both ends in the axial direction of the discharge space. Therefore, the amount of the metal halide deposited on the slit can be reduced so that the change of the light emitting color of the arc tube and the generation of lighting failures can be suppressed effectively.
In the arc tube according to the second aspect of the invention, moreover, an axial distance in such a direction as to go away from the light emitting tube portion from the neck portion of the arc tube body to a tip of an almost wedge-shaped slit formed on both ends in an axial direction of the discharge space is set to have a value of 0.5 mm or less.
More specifically, since the axial distance from the neck portion to the tip of the slit is very short, the volume of the slit can be reduced. Consequently, the amount of the metal halide deposited on the slit can be reduced so that the change of the light emitting color of the arc tube and the generation of lighting failures can be suppressed effectively.
In the method of manufacturing an arc tube according to the invention, furthermore, when pinch sealing the tungsten electrode in the pinch seal portion of the arc tube body, the pinch seal is carried out by using a pair of pinchers having a step-up plane portion for forming a step-down plane portion in the pinch seal portion and causing an edge on the light emitting tube portion side in the step-up plane portion of each of the pinchers to abut on the arc tube body in a position from a position where the neck portion is to be formed by an axial distance of 1 mm or less.
More specifically, since the edge on the light emitting tube portion side in the step-up plane portion of the pincher abuts on the arc tube body in a very close position to the position where the neck portion is to be formed during the pinch seal, a sufficient pinching pressure can be applied to the tungsten electrode up to a portion close to the tip portion thereof. Consequently, since the volume of the almost wedge-shaped slit formed on both ends in the axial direction of the discharge space can be reduced, the amount of the metal halide deposited on the slit can be decreased. As a result, the change of the light emitting color of the arc tube and the generation of lighting failures can be suppressed effectively.